Some game designs define limits on the realtime duration of a game. For
instance, the game might be set to end in one hour, a single turn might
be limited to always last at most 2 minutes, or your side might be
limited to 15 minutes of playing time, in the manner of a chess clock.
If such limits are in effect, your display should be able to show you
how much time you have left at any moment; pay attention!

When you run out of time, you are not automatically taken out of the
game, but you can no longer do anything with your units. Units that
already have plans will continue to act on them.

You can see what the limits are by looking at the "game design" node
of the online help.